This invention relates generally to spacecraft communication systems and, in particular, to spacecraft communication systems t hat have at least one spacecraft that receives uplink signals from a number of ground station transmitters located within beams, and that frequency shifts and retransmits the received uplink signals to receivers located within the same beams.
The use of a geosynchronous orbit satellite to broadcast television signals to terrestrial receivers is well known in the art. By example, reference can be had to the following two publications: xe2x80x9cFlight Hardware Test Results Obtained o n High Power Equipment and on the Repeater Subsystem of 12 GHz DBSxe2x80x9d, W. Liebisch et al., 86-0646 AIAA, pp. 266-274, 1986; and xe2x80x9cThe Thermal Control System of the German Direct Transmitting Communication Satellite TV-SATxe2x80x9d, Kreeb et al., AIAA 8th Communications Satellite Systems Conference, Apr. 20-24, 1980.
A number of problems are presented in the design of a high performance satellite communications system that provides, for example, television service to terrestrial receivers spread over a large geographical area. In such a system a number of different ground stations, each associated with a different locale and demographic market, may each transmit an uplink signal that is intended to be received by a spacecraft, such as a geosynchronous orbit satellite, and then transmitted, through one or more transponder channels, from the spacecraft to television receivers within the locale served by the ground station. For example, one ground station may serve the New York City area, another may serve the St. Louis area, while another serves the Salt Lake City area. Each ground station can provide one or more television channels, and is considered to be located within a particular spacecraft beam. More than one ground station can be serviced by a particular beam.
As can be appreciated, in such a system the size and hence downlink power requirements of each service area may differ significantly. That is, for a predetermined amount of RF power at the ground, more spacecraft transmitter power is required for a large beam than for a small beam. Furthermore, and in order to maximize the total number of ground stations that can be serviced, the spacecraft will require a significant number of uplink receivers, as well as a significant number of downlink power amplifiers, typically implemented as Travelling Wave Tube Amplifiers (TWTAs). In addition, it is important to provide some capability to control the transmission power so as to compensate for localized signal impairments, typically rain attenuation, that may be experienced at any given time in one locale but not in others.
It is known to provide gain and RF power control of transponder channels on one downlink beam with one ground station, but not with gain and RF power control of different transponder channels with multiple ground stations in a downlink beam.
In accordance with the prior art, and referring to FIG. 1A, a spacecraft communication system may have a spacecraft 1 that uses separate TWTAs 2 that each receive a separate signal from ground stations (GSs) located within the same or different beams. By example, a first beam (beam #1) may include four ground stations (GS1-GS4) while a second beam (beam #2) may include six different ground stations (GS1-GS6). Each ground station signal is passed through a separate spacecraft transponder channel, which includes a channel amplifier circuit, shown generally as an amplifier 4, and a TWTA 2. Each channel amplifier circuit 4 may be separately gain and/or RF power controlled by the associated ground station. The outputs of the TWTAs 2 for each beam are combined in an output multiplexer (OMUX) 3 prior to transmission on the downlink to the terrestrial receivers in each regional or spot beam.
It can be realized that this conventional approach can be wasteful of power and TWTAs, as each transponder channel will typically have differing RF power requirements. If it were desired to use only one type of TWTA (e.g., a 60 W TWTA) or only two types (e.g., 60 W and 120 W), then a transponder channel that requires only 10 W of RF power will use its TWTA much less efficiently than another transponder channel that requires 50 W of RF power.
Further in accordance with the prior art a single size spot beam may be provided that is contiguous across the continental United States (CONUS). Alternatively, and as is exemplified by U.S. Pat. No. 4,819,227, xe2x80x9cSatellite Communications System Employing Frequency Reusexe2x80x9d to H. A. Rosen, a two-way satellite communication system can use spot beams in contiguous zones. In general, the prior art requires either more satellites or larger spot beam spacing, using a single size of spot beams, to obtain a required performance. The prior art may as well use more antennas interlaced over the CONUS area, with larger feed spacings and thus require more area on the satellite.
It is also known from the prior art to provide as many receivers as the total number of transponder channels, or as many as the number of feeds/beams, and to have each receiver translate its associated transponder channel or feed/beam to the appropriate downlink channel frequencies. Referring to FIG. 1C, the prior art teaches a system that uses either a single receiver 7 for one transponder or a single receiver 7 for one feed or one beam. As was also the case for FIG. 1A, each GS signal may originate from a separate geographical area (e.g., from ground stations located in different urban areas).
As can be appreciated, and as was also the case for FIG. 1A, the prior art approaches are not efficient with regard to spacecraft power consumption, weight, and/or payload utilization.
It is a first object and advantage of this invention to provide an improved satellite communications system wherein a plurality of satellite transponder channels are selected so as to combined and amplified by a single linearly driven high power amplifier, such as one TWTA or multiple paralleled TWTAs.
It is another object and advantage of this invention to provide a technique for summing a plurality of uplink satellite transponder channels into a linearly driven single TWTA or multiple paralleled TWTAs, or some other type of high power RF amplifier, and to then separate the amplified transponder channels into a plurality of distinct downlink feeds and/or spot or regional beams.
Certain of the foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
In accordance with this invention there is provided a satellite communication RF power control system to deliver digital data, such as digital television data, from multiple ground stations to feeds and/or spot or regional beams.
The teachings of this invention relate to a satellite communication RF power sharing system to deliver digital data from multiple spot or regional beams via a combination of input combiners and output splitters, and by amplifying a plurality of signal/transponder channels with one high power amplifier (HPA), such as one or more TWTAs or solid-state power amplifiers. For spot or regional beams with low data traffic and low required RF downlink power, this aspect of the invention can combine multiple feeds and/or beams to one HPA or TWTA, and can then use a reverse output multiplexer (OMUX) to split the output of the HPA or TWTA to the designated feeds and/or beams. In the context of this invention one or more feeds can be form one beam.
A previous known solution would employ many HPAs or TWTAs with different power levels to deliver downlink power for each spot or regional beam. This conventional approach would thus require a large set of low power and high power HPAs or TWTAs on the satellite, whereas the teaching of this invention enables a reduction in and/or an elimination of the low power HPAs or TWTAs, which typically exhibit lower efficiency. That is, by combining a plurality of transponder channels that would conventionally require a plurality of low power TWTAs (or some other type of RF power amplifier) into one higher power TWTA, and then separating the amplified transponder channels, a number of the low power TWTAs can be completely eliminated from the spacecraft, thereby conserving payload weight, volume and power consumption.
This aspect of the invention employs an analysis of how many transponder channels can be combined and power amplified in a single HPA or TWTA, or multiple paralleled TWTAs, and determines which feeds and/or beams have low data traffic and low required downlink power that makes then suitable candidates for combining their feeds and/or beams into a linearly driven HPA or TWTA, or multiple paralleled TWTA power amplifier. The reverse OMUX 46 is used to separate the combined feeds and/or beams to the separate individual downlinks after the power amplification of the HPA or TWTA, or multiple paralleled TWTAs.
This invention thus provides a satellite communication system that includes at least one spacecraft in geosynchronous orbit that provides a plurality of beams on the surface of the earth, and a plurality of ground stations individual ones of which are located in one of the beams for transmitting uplink signals to one of the spacecraft. The spacecraft has a plurality of receivers for receiving a plurality of the uplinked signals from ground stations, a frequency translator for translating the received uplink signals to a transmission frequency of a plurality of downlink signals, and a plurality of transmitters for transmitting the plurality of downlink signals within the same beams as the corresponding uplink signals. Each transmitter includes a combiner for combining together a plurality of frequency translated signals and a power amplifier, such as a TWTA, for amplifying the combined plurality of frequency translated signals. The spacecraft further includes an output splitter for separating the amplified combined plurality of frequency translated signals into a plurality of downlink signals in a plurality of feeds and/or beams.
Individual ones of the plurality of frequency translated signals are selected such that a sum of a maximum downlink RF power will not exceed the power handling capability and linearity of a single TWTA or multiple paralleled TWTAs.